1. Field of the Invention
This invention relates to means for reducing the generation of static electricity, and more particularly, relates to a unique and novel chair mat having a construction such that when castered furniture, such as a secretary's chair, rolls across the surface of this chair mat the generation of static electricity is greatly reduced.
2. Background of the Prior Art
With the advent of computers and word processing equipment the static generation caused, especially during cold weather, by the movements of personnel and by the rolling of castered chairs across floor coverings has caused many problems. The static electricity may cause computers or their associated terminals to malfunction, or the memory systems of some word processors may be disturbed or completely lose the material stored therein.
It is well known in the prior art that chair mats usually made of plastic material are placed on top of carpeting in offices so that when castered furniture rolls across the surface of the mat it does so more easily than rolling across the surface of a carpeted floor. Many offices throughout this country use such chair mats.
In order to avert the static problems mentioned above, one of the prior art items that was produced was a chair mat made of plastic with a high loading of carbon black material. The electrical conductivity provided by the carbon proved effective in controlling static by (1) suppressing the generation of static arising from motions of persons or chairs on the mat, and (2) dissipating static charges carried by personnel approaching from an area having static-prone floor covering. However, this type of chair mat had serious drawbacks in that it was made of a material which curled under continuous pressure and was black in color, thereby not blending with the decor of the office nor, in particular, with the carpeted floor covering. A further drawback was the apparent necessity of electrically grounding such black prior art mat to insure its static control efficacy, said grounding having been prescribed by every supplier of such a mat. It is a demonstrable fact that, in the great majority of office applications, a properly designed ungrounded mat can effectively control static so that the performance of adjacent sensitive electronic equipment is not affected thereby.
To perform effectively, an anti-static mat must achieve two objectives: (1) it must suppress the generation of static voltages below the level which will cause problems with electronic equipment, and (2) it must dissipate static charges which may be carried by personnel coming from a static-prone area and approaching the electronic equipment to be safeguarded. As to the first of these, static generation arises from ordinary movements of people and furniture upon a floor or floor covering. The magnitude of static charge depends on the materials involved, the nature of the motion and the relative humidity. Even on a theoretical basis, the presence or absence of an electrical connection between the mat and ground has no bearing on static generation.
An effective anti-static mat must be conductive to a degree. When an approaching charged person steps onto the mat, the charge flows from his shoe soles into the mat. If the mat is grounded, then the charge is dissipated through the wire to ground. Voltage on the person and on the mat is quickly reduced to zero. However, if the mat is not grounded, then a charged person stepping onto the mat shares that charge with the mat, but not equally. The mat assumes the greatest portion of the charge, since its capacitance to ground far exceeds that of the person. As a consequence, the voltage on the person drops quickly to a small fraction of its initial value, so that any static discharge which may then occur (from the person's finger, for example) contains so little energy that no electronic equipment is disturbed. Furthermore, since even an ungrounded mat is never perfectly isolated from ground, any residual charge on the mat leaks continuously to ground through the underlying floor covering and approaches zero in a relatively short period of time. Grounding of an anti-static mat merely offers a slight, and usually inconsequential, improvement in the speed with which body voltage is reduced to a safe level.
The conclusion which may be drawn from the foregoing, and which has been borne out in thousands of actual cases, is that an anti-static mat which incorporates suitable conductivity in its face and through its thickness may offer effective static control with all classes of sensitive electronic equipment under most circumstances, even if the mat is not grounded.